1. Field of the Invention
The present invention relates to apparatus in the manufacture of flat glass wherein the glass is formed while being supported on the surface of a pool of molten metal following its delivery thereto as a stream of molten glass flowing onto the molten metal over a refractory surface which extends into the molten metal. More particularly, this invention relates to a combination of elements comprising means for supporting molten glass during its delivery onto such a pool of molten metal.
2. Brief Description of the Prior Art
It is known that molten glass can be delivered onto molten metal and thereafter formed into a continuous ribbon or a sheet of glass, according to the teachings of Heal, U.S. Pat. No. 710,357; of Hitchcock, U.S. Pat. No. 789,911; of Pilkington, U.S. Pat. Nos. 3,083,551 and No. 3,220,816; and of Edge and Kunkle, U.S. Pat. No. 3,843,346. In all of the prior art molten glass is delivered over some rigid element, usually a refractory member, onto molten metal inside a forming chamber. In the practice disclosed by Pilkington the molten glass is delivered through a long, narrow canal and over a lip which is spaced above the pool of molten metal. The molten glass then falls onto the molten metal and spreads rearwardly and outwardly, as well as advancing in a forward manner along the surface of the pool of molten metal. In the other described methods, molten glass is delivered over a refractory wall, bridge or threshold and, without free fall or rearward flow, is deposited directly onto a pool of molten metal. It then advances along the surface of the pool of molten metal in the same general path of flow which it experienced during delivery. It is thereafter cooled and attenuated to form a dimensionally stable, continuous sheet or ribbon or glass. The continuous sheet or ribbon of glass is then removed from the forming chamber.
The preferred delivery facility for delivering a stream of molten glass from the refiner or conditioner of a glassmaking furnace to a glass forming chamber, according to the method of Edge and Kunkle, is shown and described in the following patents: U.S. Pat. Nos. 3,854,922, No. 3,898,069 and No. 3,884,665.
U.S. Pat. No. 3,854,922 to Sensi and Wehner shows a glass delivery apparatus in which a bottom threshold is mounted over the front basin wall of a glassmaking furnace and urged against the upstream or inlet end of a glass forming chamber to serve as a common wall between the glassmaking furnace and the glass forming chamber and to povide a glass support element over which a stream of glass may be delivered for forming. Side members or jambs extend upwardly at the ends of the threshold and a roof or flat arch overlies the assembly and faces it from above. A metering member or tweel extends transversely across the delivery facility in substantial alignment with the theshold. The tweel, the side members and the theshold define an opening through which molten glass may be delivered for forming.
U.S. Pat. No. 3,898,069 to Cerutti and Gulotta discloses and claims a positive containment threshold having a transversely disposed, impervious member extending through the threshold and an extended portion of a forming chamber extending under the threshold and upwardly along the sides of the delivery facility.
U.S. Pat. No. 3,884,665 to Edge and Kunkle, discloses and claims a threshold having a particularly preferred shaped for the glass support member in such a delivery facility and discloses particularly preferred materials from which such a threshold may be made. A preferred threshold has an upwardly-facing convex upper glass-supporting surface so that there is a minimum of glass refractory contact during the delivery of molten glass over the threshold. The downstream face of the threshold is in contact with molten metal in the forming chamber so that there is no falling or pouring of molten glass onto the molten metal. There is, rather, a smooth flow of glass through the delivery facility and onto the molten metal. Depending upon the elevation of the upper surface of a pool of molten metal in the forming chamber, molten glass flowing over the threshold may flow in a perfectly horizontal fashion from the uppermost portion of the threshold onto and along the surface of the pool of molten metal; or, if the elevation of molten metal is a bit lower, the molten glass may flow in conformity to the threshold shape down a slope and onto the surface of the molten metal where it is advanced along the molten metal while being formed into a continuous sheet or ribbon of glass. In either event, the delivery of the glass is without free fall and rearward flow, as has been characterized to be substantially horizontal flow in U.S. Pat. No. 3,843,346.
While the delivery facilities described and claimed in these patents have been satisfactorily employed to make flat glass without offensive linear surface defects, it has been noted that gases residing or accumulating along the interior of the forming chamber casing or between the refractory liner and molten metal in the forming chamber may enter the molten glass just as it is delivered across the threshold surface-molten metal interface causing scattered bubble-like deformations in the undersurface of glass being produced. The present invention is directed to an apparatus and method for substantially avoiding the occurrences of such deformations in the underside of glass being produced while maintaining substantially lamellar flow conditions within the stream of glass being delivered to and advanced along the molten metal for forming in accordance with the teachings of Edge and Kunkle.